Steering apparatuses for steering motor vehicles are often equipped with steering-assistance devices in order to reduce the force which is necessary for operating a steering wheel of the steering apparatus, in particular when the motor vehicle is stationary or travelling at low speeds. In addition to hydraulically operated steering-assistance devices, the prior art also discloses electrical steering-assistance devices which can be realized in an electrohydraulic or electromechanical manner. An electromechanical steering-assistance device has an electric actuating drive which acts on a torsion bar of the steering apparatus and can be driven by means of a control device. As a result, the actuating motor is able to provide auxiliary forces in order to assist steering wheel movements by the driver in this way.
The steering apparatus of a motor vehicle is subject to numerous interfering influences during operation, it being possible for these interfering influences to have a disadvantageous effect on the steering and traveling behavior of the motor vehicle. In the case of an electromechanical steering-assistance device, high-frequency vibrations with a relatively large amplitude can be produced on the torsion bar if there is no suitable steering-stabilization function. This effect occurs particularly when the torsion bar of the steering apparatus is relieved of loading (for example when steering back with an only low manual torque exerted by the driver). An existing steering-stabilization function which is dependent on the torsion bar torque and can be implemented, in particular, in the control device for the actuating motor can effectively prevent vibrations of this kind, but at the same time suppresses excitation phenomena which act on the steering apparatus on account of the condition of the carriageway. This in turn results in insufficient steering feedback which can be perceived by the driver and can have a disadvantageous influence on the steering sensation. It would therefore be desirable for the steering-stabilization function to be activated only when actual instabilities occur.
DE 10 2008 036 001 A1 discloses a method of the kind mentioned in the introductory part for operating a steering apparatus comprising an electric steering-assistance device. In this method, interference variables which are caused by the condition of the carriageway and act on the steering apparatus are ascertained by a frequency analysis. When the ascertained interference variable exceeds a prespecified amplitude, the electric steering-assistance device generates a compensation torque which can counteract an interference-induced torque which is generated by the interference variable and acts on the steering apparatus—in particular on the steering wheel.